This invention relates to a coding device for use in a digital communication system and, more particularly, to a predictive coding device to which a transmission line is effectively available.
In digital communication systems, analogue signals are coded into digital signals by waveform coding such as pulse code modulation (PCM). Another type of waveform coding is predictive coding, for example, differential PCM (DPCM), adaptive DPCM (ADPCM), and so on. The ADPCM is disclosed in CCITT Recommendation G.721 of the title of "32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM)."
Such a predictive coding system includes a coding device in a transmitting station. The coding device codes an input signal into a coded signal which should be transmitted to the transmission line. The coding device comprises a subtractor for subtracting a local decoded signal from the input signal to produce a difference signal. The difference signal is coded into the coded signal by a quantizer. The quantizer may be an adaptive quantizer. The coded signal is locally decoded into the local decoded signal by a local decoder. The predictive coding system further includes a decoding device in a receiving station. The decoding device has a structure which is identical to the local decoder of the coding device and produces a decoded signal in response to the coded signal received from the coding device through the transmission line.
In the predictive coding system described above, the decoding device obtains a current element of the decoded signal from not only a current element of the coded signal but also a series of previous elements of the coded signals which have been received before the current element of the coded signal. Therefore, the decoding device must always continuously receive the series of previous elements which are identical with signal elements of the coded signal used in the local decoder of the coding device. Otherwise, operation of the decoding device will be different from that of the local decoder, which makes correct reproduction difficult in the decoding device.
By the way, a digital communication system is known such that the coding device is selectively connected to the transmission line. More particularly, the coding device is connected selectively to the transmission line for a predetermined number of channels and is disconnected therefrom for one or more channels in excess of the predetermined number. Such selective connection may be carried out, for example, by use of a digital speech interpolation (DSI) technique which will later be described later in more detail. In such a digital communication system, the decoding device receives a series of coded signal elements which are different from the signal elements used in the local decoder of the coding device when the coding device is disconnected from the transmission line.